Low volume fraction metal matrix composites are just beginning to gain market acceptance. Aluminum matrix composites with silicon carbide are now being used in a variety of applications. These materials provide higher stiffness and increased wear resistance and are being used to replace aluminum, steel, and cast iron components. Some of these components include extrusions, forgings, and castings for industrial and automotive applications. Components such as pump housings, brake rotors, engine blocks, cylinder liners, and bicycle frames are some of the current aluminum/silicon carbide composite applications.
Low volume fraction metal matrix composite materials are typically produced by one of three methods, all of which include producing an ingot of material which is solidified and later remelted and formed into a component. The three methods used to produce these ingots are stirring, powder metallurgy, and infiltration.
Aluminum low volume fraction composites are being produced by Duralcan and other companies by mixing silicon carbide particles in a crucible using a low-vortex stirrer as described in U.S. Pat. No. 4,865,806, incorporated by reference herein. As disclosed in this patent, the material is mixed in a crucible in an evacuated atmosphere between 15 minutes and 2 hours, and then the stirring head is replaced with a casting head. After the casting head has been put in place, the surface of the liquid metal is pressurized with a gas to force the liquid composite mixture into a water-cooled ingot mold. Later the ingot is remelted and restirred and cast in the shape of a component.
This is a batch-type operation which requires stopping and opening the mixing machine at various times during the mixing to scrape reinforcement off the side walls and to switch heads. All the material must be mixed first. Then the mixing is stopped and ingots are formed, the mixed material is used up, and a new batch is made. In this process, the size of the batch is controlled by the size of the crucible. Typically, 100 lbs. or less of material is produced per batch due to the difficulty in uniformly mixing large volumes of material.
An alternative method used to produce composite ingots is powder metallurgy. For example, powder aluminum can be mixed with silicon carbide particulate and then cold and/or hot pressed to form an ingot. This method is currently being used by Alcoa and DWA, Inc. to produce low volume fraction composite ingots. These ingots may then be extruded or remelted for casting.
In the infiltration method to produce composite ingots, different processes, including gas pressure infiltration and pressureless infiltration, may be used a number of different ways to create a composite ingot. Infiltration can be used to create a highly loaded composite ingot which can be diluted in a melt to the desired particle loading. An alternative method is to infiltrate the reinforcement located in the bottom of a crucible and then stir the melt to cause the infiltrated reinforcement to disperse.
All of the current methods of producing low volume fraction metal matrix composite components involve first creating solidified low volume fraction ingots in a batch process, remelting them, and then forming a component. Typically, after solidification and formation of a composite ingot, the composite ingots are then remelted in a crucible and stirred to keep the reinforcement dispersed. The material is then cast into a mold to produce one or more components. After the crucible of material is spent, the casting process is stopped, and a new batch is made by melting additional composite ingots.
This batch-type process requires a large amount of labor and equipment to produce low or medium volumes of components. This process is not ideally suited for high volume continuous component production. Also, the two heating cycles, one to produce the composite ingot, and the other to reheat and cast, requires a high energy consumption, especially in the case of aluminum, which has a high heat of fusion. Current processes are therefore expensive due to the high energy consumption and too slow for mass production due to the batching operations required. Also, only a few materials are available from suppliers which gives the component producer little flexibility in choice of material systems.